Several receptor complexes that play a role in leukocyte activation and inflammatory responses (Gingras et al. (2001) Mol. Immun. 38:817-824) are formed by the non-covalent association of the transmembrane adaptor glycoprotein DAP12 with receptors of the Ig superfamily (Bouchon et al. (2000) J. Immunol. 164:4991-4995; Dietrich et al. (2000) J. Immunol. 164:9-12) or the C-type lectin superfamily (Bakker et al. (1999) PNAS U.S.A. 96:9792-9796). These associations are formed by the interaction of a negatively charged amino acid residue (aspartic acid) located in the DAP12 transmembrane domain with a positively charged amino acid residue (lysine) located in the transmembrane domain of these receptors (Gingras et al. (2001) Mol. Immun. 38:817-824).
DAP12 is a disulfide-bonded, homodimeric type I transmembrane glycoprotein containing an immunoreceptor tyrosine-based activation motif (ITAM) located in its intracellular domain (Lanier, et al. (1998) Nature 391:703-707; WO 99/06557; Campbell and Colonna (1999) Int. J. Biochem. Cell Biol. 31:631-636; Lanier and Bakker (2000) Immunol. Today 21:611-614). The importance of DAP12 relies on the ITAM domain (Gingras et al. (2001) Mol. Immun. 38:817-824). Because the intracellular domain of the receptors of the Ig superfamily (Bouchon et al. (2000) J. Immunol. 164: 4991-4995; Dietrich et al. (2000) J. Immunol. 164:9-12) and the C-type lectin superfamily (Bakker et al. (1999) PNAS U.S.A. 96:9792-9796) that non-covalently associate with DAP12 are too short to allow interaction with other molecules, the DAP12 cytoplasmic domain constitutes the signaling subunit of these receptor complexes. Upon engagement of the receptor ligand-binding subunit, the DAP12 cytoplasmic ITAM is phosphorylated by Src kinases. The ITAM of DAP12 then interacts with Syk cytoplasmic tyrosine kinases, which initiates a cascade of events that leads to activation (Lanier et al. (1998) Nature 391:703-707; Campbell and Colonna (1999) Int. J. Biochem. Cell Biol. 31:631-636; Lanier and Bakker (2000) Immunol. Today 21:611-614).
DAP12 is expressed in monocytes, macrophages, natural killer (NK) cells, granulocytes, dendritic cells and mast cells, where it provides signaling function for at least eight distinct receptors (Gingras et al. (2001) Mol. Immun. 38:817-824; Lanier and Bakker, (2000) Immunol. Today 21:611-614). The myeloid receptor of the C-type lectin superfamily associated with DAP12 is Myeloid DAP12-associating Lectin-1 (MDL-1), a type II transmembrane protein. MDL-1 was the first DAP12 associating molecule to be identified and cloned (Bakker et al. (1999) PNAS USA 96(17):9792-9796). It is expressed exclusively in monocytes and macrophages (Bakker et al. (1999) supra) as well as on other myeloid cell types such as, neutrophils and dendritic cells. The presence of a negatively charged residue in the transmembrane domain of DAP12 precludes its cell surface expression in the absence of a partner receptor, such as MDL-1, which has a positively charged residue in its transmembrane domain. However, DAP12 alone is not sufficient for its expression and function at the cell surface. Thus, the combination of a DAP12-associating molecule, such as MDL-1, and DAP12 may account for transmitting a particular physiological signal via DAP12 (Nochi et al. (2003) Am. J. of Pathology 162:1191-1201).
The need exists for improved methods and compositions for the treatment of immune mediated disorders, in particular, infectious diseases, by use of agents that modulate DAP12 signaling through the use of agonists against MDL-1. Preferably, such agonists would have a high affinity for the target molecule, and would be able to stimulate the MDL-1 mediated DAP-12 signaling at relatively low doses. Preferably, such methods and compositions would be highly specific for MDL-1, and not interfere with the activity of other activating or inhibitory receptors, such as TREM-1. Preferably, such methods and compositions would employ agonists suitable for modification for or the delivery of cytotoxic payloads to target cells, but also suitable for non-cytotoxic uses. Preferably, such methods and compositions would employ antibodies modified to limit their antigenicity when administered to a subject in need thereof.